给button增加下划线

- (void)setUnderLineForButton:(UIButton *)btn withTitle:(NSString *)title{

    //利用富文本的方式增加button下划线

    NSMutableAttributedString *str = [[NSMutableAttributedString alloc]initWithString:title];

    NSRange strRange = {0,[str length]};

    [str addAttribute:NSUnderlineStyleAttributeName value:[NSNumber numberWithInteger:NSUnderlineStyleSingle] range:strRange];

    [btn setAttributedTitle:str forState:UIControlStateNormal];

}